A popular method for generating Ultra-Wideband (UWB) Signals uses a technique known as Impulse Radio (IR) as described in Moe Z. Win and Robert A. Scholtz, ‘Impulse Radio: How it Works’, IEEE Communications Letters, Vol. 2, No. 2, February 1998, and Moe Z. Win and Robert A. Scholtz, ‘Ultra-Wide Bandwidth Time-Hopping Spread-Spectrum Impulse Radio for Wireless Multiple-Access Communications’, IEEE Transactions on Communications, Vol. 48, No. 4, April 2000. This technique involves using a low duty cycle pseudo randomly time hopped pulse train of short (sub-nanosecond) mono-cycles to create a signal with an Ultra Wide-Band spectrum. In a UWB communications system the receiver needs to have an accurate idea of when the pulses will arrive in order to receive the signal correctly, i.e. the receiver has to be synchronised to the received pulse train.
The combination of the low duty cycle pulse train and the pseudo random time hopping code means that it can be difficult and time consuming to find the signal. One of the most reliable method of searching for the signal is to choose a time offset resolution which is smaller than the pulse and perform an exhaustive search for all time offsets and all hopping code offsets. However, this search takes the longest to perform and may not be suitable for some applications. Some searches which examine a subset of the search space and use a threshold technique have been proposed but these methods can exhibit low performance in certain scenarios and have a variable search time which can approach that of the exhaustive search in worst case situations.
Current technology employs a single Time-Integrating Correlator (TIC) as described in Burke, B. E.; Smythe, D. L.; ‘A CCD time-integrating correlator’, Solid-State Circuits, IEEE Journal of, Volume: 18 Issue: 6, December 1983 Page(s): 736-744. Typically the pulse train is found by triggering the TIC for all time and code offset combinations to perform an exhaustive search for the signal as described in Yao Ma; Chin, F.; Kannan, B.; Pasupathy, S.; ‘Acquisition performance of an ultra wide-band communications system over a multiple-access fading channel’, Ultra Wideband Systems and Technologies, 2002. Digest of Papers. 2002 IEEE Conference on, 2002 Page(s): 99-103. Some methods use a reduced search area combined with a threshold operation as described in Homier, E. A.; Scholtz, R. A.; ‘Rapid acquisition of ultra-wideband signals in the dense multipath channel’, Ultra Wideband Systems and Technologies, 2002. Digest of Papers. 2002 IEEE Conference on, 2002 Page(s): 105-109.
A system which aims to acquire fast acquisition of UWB IR time hopped sequences by ensuring that all absolute time differences between adjacent pulses is unique is described in Leanard S. Haynes and Mark D. Roberts, “Method and System for Fast Acquisition of Pulsed Signals”, US Patent 2002/0018514, February 2002. However this method seems impractical in the presence of multi-path and multiple access interference. A simple way of reducing the amount of time taken to perform synchronisation is to not observe the code for its full length but to only observe a smaller subset as described in Larry W. Fullerton, “Fast locking mechanism for channelized ultrawide-band communications”, U.S. Pat. No. 5,832,035, November 1998.
The problem with using an exhaustive search is that, although the performance is likely to be the best, the search times can become unacceptably high for some applications.
Some methods have been proposed to decrease the time by using a reduced search in combination with a threshold function. However, choosing the correct threshold for dynamic, and unknown, signal to noise ratio (SNR) and Multiple Access Interference (MAI) scenarios may be difficult which will compromise performance. Also these methods, although reducing the average search time, can exhibit high instantaneous search times (approaching that of the exhaustive search time) in worst case conditions.
Moe Z. Win and Robert A. Scholtz, “Impulse Radio: How It Works”, IEEE Communications Letters, Vol. 2, No. 2, February 1998 describes impulse radio, a form of ultra-wide bandwidth (UWB) spread-spectrum signalling, that has properties that make it a viable candidate for short-range communications in dense multipath environments. This paper describes the characteristics of impulse radio using a modulation format that can be supported by currently available impulse signal technology and gives analytical estimates of its multiple-access capability under ideal multiple-access channel conditions.
Moe Z. Win and Robert A. Scholtz, “Ultra-Wide Bandwidth Time-Hopping Spread-Spectrum Impulse Radio for Wireless Multiple-Access Communications”, IEEE Transactions on Communications, Vol. 48, No. 4, April 2000 describes attractive features of time-hopping spread-spectrum multiple-access systems employing impulse signal technology, and emerging design issues are described. Performance of such communications systems in terms of achievable transmission rate and multiple-access capability are estimated for both analog and digital data modulation formats under ideal multiple-access channel conditions.
Yao Ma; Chin, F.; Kannan, B.; Pasupathy, S.; “Acquisition Performance Of An Ultra Wide-Band Communications System Over A Multiple-Access Fading Channel”, Ultra Wideband Systems and Technologies, 2002. Digest of Papers. 2002 IEEE Conference on, 2002 Page(s): 99-103 describes how ultra wide-band time-hopping communications are expected to be a practical scheme in the near future, however that the acquisition analysis of the UWB signals over multipath channels has not been adequately addressed.
Homier, E. A.; Scholtz, R. A.; “Rapid Acquisition of Ultra-Wideband Signals in the Dense Multipath Channel”, Ultra Wideband Systems and Technologies, 2002. Digest of Papers. 2002 IEEE Conference on, 2002 Page(s): 105-109 describes efficient serial search strategies which are shown to reduce drastically the mean acquisition time for UWB signals in a dense multipath environment. Inherent in traditional serial search problems is the assumption that only a single bin or a small number of consecutive bins can properly terminate the search. This assumption leads to search strategies which tend to be linear in nature, e.g., a linear sweep of the uncertainty region. Because of the dense multipath channel present in most UWB systems, this assumption is invalid as seen by the channel's relatively large delay spread. A generalized analysis of various search algorithms is presented based on a Markov chain model of a simple single-dwell serial search. The results from this analysis reveal that the linear search has a considerably larger mean acquisition time than the more efficient search strategy termed the bit reversal search.
Burke, B. E.; Smythe, D. L.; “A CCD Time-Integrating Correlator”, Solid-State Circuits, IEEE Journal of, Volume: 18 Issue: 6, December 1983 Page(s): 736-744 describes a CCD binary-analog time-integrating correlator that has been designed and operated at 20 MHz clock rate. The 32-channel device is capable of integration periods in excess of 25/spl mu/s or 500 clock periods, equivalent to a time-bandwidth product of 250. The device architecture is based on charge-domain signal processing for high-speed operation and does not required on-chip logic for storage of the binary reference. The device is tailored for weak signal applications, and a new charge skimming circuit has been devised which allows the small portion of the integrated charge containing the correlation function to be separated from the large register by tenfold. The correlator has a stationary pattern noise which can be eliminated with simple postprocessing, yielding a dynamic range of 67 dB.
Leanard S. Haynes and Mark D. Roberts, “Method and System for Fast Acquisition of Pulsed Signals”, US Patent 2002/0018514, February 2002 describes a method and system for fast acquisition of pulsed radio signals where said signals comprised uniquely-spaced pulses or symbols. The receiver searches for and detects two ore more symbols, determines the interval or intervals between the symbols, and because the intervals are unique, the symbols or symbol sequences are identified. Identification of the symbols and their respective timing within the signal allows the receiver to adjust its timing to synchronize with the full signal.
Larry W. Fullerton, “Fast locking mechanism for channelized ultrawide-band communications”, U.S. Pat. No. 5,832,035, November 1998 describes a receiver for acquisition and lock of an impulse radio signal comprising an adjustable time base to output a sliding periodic timing signal having an adjustable repetition rate, and a decode timing modulator to output a decode signal in response to the periodic timing signal. The impulse radio signal is cross correlated with the decode signal to output a baseband signal. The receiver integrates T samples of the baseband signal and a threshold detector uses the integration results to detect channel coincidence. A receiver controller stops sliding the time base when channel coincidence is detected. A counter and extra count logic, coupled to the controller, are configured to increment or decrement the address counter by a one or more extra counts after each T pulses is reached in order to shift the PN code modulo for proper phase alignment of the periodic timing signal and the received impulse radio signal.